Use breakpad_getcontext on all Linux platforms missing getcontext

getcontext is also not available on musl libc, so generalize
breakpad_getcontext so it can be used as a fallback for non-Android
platforms as well.

On x86_64 and i386, ucontext_t uses an Android-specific offset for
storage of FP registers, since its sigset_t differs in size. So,
make the definition of MCONTEXT_FPREGS_MEM and UCONTEXT_FPREGS_MEM_OFFSET
conditional on whether we are building for Android.

On glibc and musl, signal.h and asm/sigcontext.h can't be included
together, so in breakpad_context_unittest.cc, only compare the libc
and kernel _fpstate when on Android.

Bug: google-breakpad:631
Change-Id: If81d73c4101bae946e9a3655b8d1c40a34ab6c38
Reviewed-on: https://chromium-review.googlesource.com/c/breakpad/breakpad/+/2102135
Reviewed-by: Mike Frysinger <vapier@chromium.org>

src/client/linux/handler/exception_handler.cc

@@ -87,6 +87,7 @@
 #include <vector>
 
 #include "common/basictypes.h"
+#include "common/linux/breakpad_getcontext.h"
 #include "common/linux/linux_libc_support.h"
 #include "common/memory_allocator.h"
 #include "client/linux/log/log.h"

src/client/linux/microdump_writer/microdump_writer_unittest.cc

@@ -40,6 +40,7 @@
 #include "client/linux/handler/exception_handler.h"
 #include "client/linux/handler/microdump_extra_info.h"
 #include "client/linux/microdump_writer/microdump_writer.h"
+#include "common/linux/breakpad_getcontext.h"
 #include "common/linux/eintr_wrapper.h"
 #include "common/linux/ignore_ret.h"
 #include "common/scoped_ptr.h"

src/client/linux/minidump_writer/minidump_writer_unittest.cc

@@ -42,6 +42,7 @@
 #include "client/linux/minidump_writer/linux_dumper.h"
 #include "client/linux/minidump_writer/minidump_writer.h"
 #include "client/linux/minidump_writer/minidump_writer_unittest_utils.h"
+#include "common/linux/breakpad_getcontext.h"
 #include "common/linux/eintr_wrapper.h"
 #include "common/linux/file_id.h"
 #include "common/linux/ignore_ret.h"