build: armv6-vfp.mk set arch to armv6k

arm1136 has some variants:
r1p0            -> armv6
r1p1 or higher  -> armv6k

msm7x27's core is r1p1 or higher so it supports full armv6k
instruction set

In armv6-vfp.mk we set mcpu flag to arm1136jf-s, but gcc doesn't
know that our core is r1p1 or higher so it thinks[1] the target arch
to be armv6

Unfortunately there is no way to set arm1136 variant r1p1 or higher
through the mcpu flag, thus we should use the march flag and set it
to armv6k

[1]
ldrexb (load exclusive byte) is a armv6k+ instruction:

mm7@userver:~$ echo "ldrexb r0, [r1]" > test.s
mm7@userver:~$ ./arm-eabi-as test.s -mcpu=arm1136jf-s
test.s: Assembler messages:
test.s:1: Error: selected processor does not support `ldrexb r0,[r1]'
mm7@userver:~$ ./arm-eabi-as test.s -march=armv6
test.s: Assembler messages:
test.s:1: Error: selected processor does not support `ldrexb r0,[r1]'
mm7@userver:~$ ./arm-eabi-as test.s -march=armv6k
mm7@userver:~$

Change-Id: I1e47969b2f71a19eafdd814581357499f853cb8b
Signed-off-by: Marco Milanese <marcomilanese7@gmail.com>