ext: libmetal: Update import of libmetal from b4b5bea to a4f7630

Pull in a more recent version of libmetal, motivated by the void
pointer fixes in efc33fe.

libmetal's b4b5bea to a4f7630 spans:

a4f7630 linux: device: fix max chars to copy for the dev name
10a0d5b lib: system: freertos: mutex: change to use atomic_int ...
4e670c5 ci: Zephyr env var for toolchain
3bef6f0 cmake: Fix Zephyr library integration
13158c3 compiler: add IAR compiler file
0820a2e microblaze: suppress io 64-bit R/W access if 64 bit atomic ...
e2e5608 io: suppress io 64 bit R/W access if 64 bit atomic not ...
efc33fe io: fix compilation error for void pointers.
32ad1d2 cache: fix compilation issue
c1ade0d travis CI: update Zephyr SDK version to 0.9.3
c49d7cd travis CI: updated arm compiler ubuntu package
d279a6a travis ci: Update FreeRTOS download path
5c70105 irq: update sys_irq_restore_enable and sys_irq_save_disable
05f0cd4 cmake: Only look for doxygen package if docs enabled
b4b5bea zephyr: Introduce WITH_ZEPHYR_LIB option

At current time of writing, libmetal's HEAD is 5bc2106641.
However this failed to build in their Travis CI.
The latest passing commit is a4f7630.

Signed-off-by: Mark Ruvald Pedersen <mped@oticon.com>