[mips] add support for using the MIPS user register for TLS data.

This work, originally from Stacey Son, uses the MIPS UserReg for
reading the TLS data, and will fall back to the normal syscall path
when it isn't supported.

This code dynamically patches cpu_switch() to bypass the UserReg
instruction so to avoid generating a machine exception.

Thanks to sson for the original work, and to Dan Nelson for
bringing it to date and testing it on MIPS32 with me.

Tested:

• mips64 (sson)
• mips74k (dnelson_1901@yahoo.com) - AR9344 SoC, UserReg support
• mips24k (adrian) - AR9331 SoC, no UserReg support

Obtained from: sson, dnelson_1901@yahoo.com