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musl: build broken on Thumb, use ARM mode

Building the musl C library on Thumb (Thumb1, not Thumb2), fails with:

 {standard input}:20: Error: only lo regs allowed with immediate -- `mov fp,#0'
 {standard input}:21: Error: only lo regs allowed with immediate -- `mov lr,#0'
 {standard input}:25: Error: unshifted register required -- `and ip,a1,#-16'

Since there are no cores that we support that are Thumb1 only, use the
same solution as the one used by glibc: build the C library in ARM
mode.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>