RFC: main/musl: memcpy: use THUMB optimizations on ARMv7

For discussion:

Since Thumb is default for armv7 gcc, we need this feature. Without it, memcpy loses approx. 40% performance on every device.

Tegra 3 device - Nexus 7 (grouper/tilapia)

Without optimizations:
real    0m 6.71s
With optimizations:
real    0m 3.78s

Reference to test: #11128 (closed)

Signed-off-by: David Heidelberg david@ixit.cz