compiler.h: Raise minimum version of GCC to 5.1 for arm64

compiler.h: Raise minimum version of GCC to 5.1 for arm64

GCC versions >= 4.9 and < 5.1 have been shown to emit memory references
beyond the stack pointer, resulting in memory corruption if an interrupt
is taken after the stack pointer has been adjusted but before the
reference has been executed. This leads to subtle, infrequent data
corruption such as the EXT4 problems reported by Russell King at the
link below.

Life is too short for buggy compilers, so raise the minimum GCC version
required by arm64 to 5.1.

Reported-by: Russell King <[email protected]>
Suggested-by: Arnd Bergmann <[email protected]>
Signed-off-by: Will Deacon <[email protected]>
Tested-by: Nathan Chancellor <[email protected]>
Reviewed-by: Nick Desaulniers <[email protected]>
Reviewed-by: Nathan Chancellor <[email protected]>
Acked-by: Linus Torvalds <[email protected]>
Cc: <[email protected]>
Cc: Theodore Ts'o <[email protected]>
Cc: Florian Weimer <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Nick Desaulniers <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Catalin Marinas <[email protected]>