Fix build on gcc 4.7

Fixes:
target asm: libm <= bionic/libm/arm/e_pow.S
/tmp/cc5RHbaP.s: Assembler messages:
/tmp/cc5RHbaP.s: Error: .size expression for pow does not evaluate to a constant

Change-Id: Ie6d1056c835d0db6da923d7286cb12a60253d5ff
Signed-off-by: Evan McClain <aeroevan@gmail.com>