valgrind: build for the right MIPS ISA revision level

When Valgrind detects a 32-bit MIPS architecture, it forcibly adds
-march=mips32 to CFLAGS; when it detects a 64-bit MIPS architecture, it
forcibly adds -march=mips64. This causes Valgrind to be built always for
the first ISA revision level (R1), even when the user has configured
Buildroot for the second ISA revision level (R2).

Since R2 is backwards compatible with R1, you can run a Valgrind built
for R1 in an R2 core. This is why nobody noticed about this problem, or
at least nobody complained.

But, since (I hope) we will support R6 in Buildroot in the near future,
this problem will become very important because R6 is not backwards
compatible with R1 or R2, so building Valgrind for R1 when your target
is R6 will result in a non-working Valgrind.

Override the CFLAGS variable (which Valgrind appends to its CFLAGS) and
pass the right -march option, so they take precedence over Valgrind's
wrongfully detected value.

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>