Reapply [sanitizers] Avoid macro clash in SignalContext::WriteFlag (NFC)

D116208 may cause a macro clash on older versions of linux, where
fs.h defines a READ macro. This is resolved by switching to a more
typical casing style for non-macro symbols.

Reapplying with changes to the symbol names in various platform
specific code, which I missed previously.

Differential Revision: https://reviews.llvm.org/D118783