linux-tools: Add pfm support

pfm is a helper library used by application to program specific monitoring events.
This is commonly and often used in profiling processes and makes it easier to choose the correct event.

Example:
AutoFDO for the Kernel provides in their instructions following command:
perf record --pfm-events RETIRED_TAKEN_BRANCH_INSTRUCTIONS:k -a -N -b -c 500009 -o kernel.data -- time makepkg -s

This results currently into following:
`Error: unknown option `pfm-events' `

Since this is commonly used in many instructions add support for it, so that its easier for users and developers to use these events.

Signed-off-by: Peter Jung <ptr1337@archlinux.org>