Add package alarm/libbcm2835 for armv6h

I've added a C library specific to the Raspberry Pi that allows users to access GPIO and SPI functionality. I built and installed this package on a raspberry pi today using the PKGBUILD that I include in this commit.

I've added buildarch=16 so that the package should only build for armv6h. 

I have tried to copy the style of PKGBUILDs I've seen throughout this repo. If there is anything I can do to improve it let me know and I'll make another commit.