[AArch64] Make -mcpu, -march and -mtune case-insensitive.

[Alan Lawrence]

Small patch to make the -mcpu, -march and -mtune command-line options
case-insensitive, allowing e.g. -mcpu=CortexA57 -march=ARMv8-A.

Tested on aarch64-none-elf with no regressions; options passed onto e.g.
ld are always lowercase (as before).

OK for trunk?

--Alan

ChangeLog:
2014-01-17  Alan Lawrence  <alan.lawre...@arm.com>
        * config/aarch64/aarch64.opt (mcpu, march, mtune): Make
case-insensitive.
diff --git a/gcc/config/aarch64/aarch64.opt b/gcc/config/aarch64/aarch64.opt
index 163f34b..f5a15b7 100644
--- a/gcc/config/aarch64/aarch64.opt
+++ b/gcc/config/aarch64/aarch64.opt
@@ -88,15 +88,15 @@ Target RejectNegative Joined Enum(tls_type) Var(aarch64_tls_dialect) Init(TLS_DE
 Specify TLS dialect
 
 march=
-Target RejectNegative Joined Var(aarch64_arch_string)
+Target RejectNegative ToLower Joined Var(aarch64_arch_string)
 -march=ARCH	Use features of architecture ARCH
 
 mcpu=
-Target RejectNegative Joined Var(aarch64_cpu_string)
+Target RejectNegative ToLower Joined Var(aarch64_cpu_string)
 -mcpu=CPU	Use features of and optimize for CPU
 
 mtune=
-Target RejectNegative Joined Var(aarch64_tune_string)
+Target RejectNegative ToLower Joined Var(aarch64_tune_string)
 -mtune=CPU	Optimize for CPU
 
 mabi=