libcxx: fixups for __BIONIC__

A few header guards seem to check various libc's, but not Android's libc
(Bionic).

Bionic supports:
- arc4random
- copy_file_range
- getcwd() extension

Link: https://android-review.git.corp.google.com/c/platform/bionic/+/41445
Link: https://android.googlesource.com/platform/bionic/+/refs/heads/main/libc/SYSCALLS.TXT
Link: https://android.googlesource.com/platform/bionic/+/refs/heads/main/libc/upstream-openbsd/lib/libc/crypt/arc4random.c

Author: nickdesaulniers

libcxx/include/__config

@@ -82,8 +82,8 @@
 //      Use rand_s(), for use on Windows.
 //      When this option is used, the token passed to `std::random_device`'s
 //      constructor *must* be "/dev/urandom" -- anything else is an error.
-#  if defined(__APPLE__) || defined(__FreeBSD__) || defined(__NetBSD__) || defined(__OpenBSD__) ||                     \
-      defined(__DragonFly__)
+#  if defined(__APPLE__) || defined(__FreeBSD__) || defined(__NetBSD__) ||  \
+      defined(__OpenBSD__) || defined(__DragonFly__) || defined(__BIONIC__)
 #    define _LIBCPP_USING_ARC4_RANDOM
 #  elif defined(__wasi__) || defined(__EMSCRIPTEN__)
 #    define _LIBCPP_USING_GETENTROPY

libcxx/include/__cxx03/__config

@@ -269,8 +269,8 @@ _LIBCPP_HARDENING_MODE_DEBUG
 //      Use rand_s(), for use on Windows.
 //      When this option is used, the token passed to `std::random_device`'s
 //      constructor *must* be "/dev/urandom" -- anything else is an error.
-#  if defined(__APPLE__) || defined(__FreeBSD__) || defined(__NetBSD__) || defined(__OpenBSD__) ||                     \
-      defined(__DragonFly__)
+#  if defined(__APPLE__) || defined(__FreeBSD__) || defined(__NetBSD__) ||  \
+      defined(__OpenBSD__) || defined(__DragonFly__) || defined(__BIONIC__)
 #    define _LIBCPP_USING_ARC4_RANDOM
 #  elif defined(__wasi__) || defined(__EMSCRIPTEN__)
 #    define _LIBCPP_USING_GETENTROPY

libcxx/src/filesystem/operations.cpp

@@ -40,8 +40,9 @@
 #include <fcntl.h> /* values for fchmodat */
 #include <time.h>
 
-// since Linux 4.5 and FreeBSD 13, but the Linux libc wrapper is only provided by glibc >= 2.27 and musl
-#if _LIBCPP_GLIBC_PREREQ(2, 27) || _LIBCPP_HAS_MUSL_LIBC || defined(__FreeBSD__)
+// Since Linux 4.5 and FreeBSD 13, but the Linux libc wrapper is only provided
+// by glibc >= 2.27, musl, and Bionic.
+#if _LIBCPP_GLIBC_PREREQ(2, 27) || _LIBCPP_HAS_MUSL_LIBC || defined(__FreeBSD__) || defined(__BIONIC__)
 #  define _LIBCPP_FILESYSTEM_USE_COPY_FILE_RANGE
 #endif
 
@@ -564,7 +565,7 @@ void __create_symlink(path const& from, path const& to, error_code* ec) {
 path __current_path(error_code* ec) {
   ErrorHandler<path> err("current_path", ec);
 
-#if defined(_WIN32) || defined(__GLIBC__) || defined(__APPLE__)
+#if defined(_WIN32) || defined(__GLIBC__) || defined(__APPLE__) || defined(__BIONIC__)
   // Common extension outside of POSIX getcwd() spec, without needing to
   // preallocate a buffer. Also supported by a number of other POSIX libcs.
   int size              = 0;