Clamp too-large hwpmc callchaindepth to the maximum

If the depth requested by the user is too large, it's better to provide
the maximum than the smaller default.

Sponsored by:	The FreeBSD Foundation
Recommit r274750.
Use canonical __PIC__ flag

It is automatically set when -fPIC is passed to the compiler.

Reviewed by:	dim, kib
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D1179
Fix the powerpc64 build.

Pointy-hat to:	me
X-MFC-with:	r274735