$ clang -O2 -pg -mfentry foo.c
was adding frame pointers to all functions. This was exposed via
compiling the Linux kernel for x86_64 with CONFIG_FUNCTION_TRACER
-pg was unconditionally setting the equivalent of -fno-omit-frame-pointer,
regardless of the presence of -mfentry or optimization level. After this
patch, frame pointers will only be omitted at -O0 or if
-fno-omit-frame-pointer is explicitly set for -pg -mfentry.