PRU: Fix R3.w0 register class

PRU: Fix R3.w0 register class

TI has clarified [1] that R3.w0 is caller saved, so allow compiler to
use it. This is safe change because older GCC versions treat R3.w0 as
fixed register and never use it.

[1] https://e2e.ti.com/support/tools/ccs/f/81/t/849993

gcc/ChangeLog:

2020-05-05  Dimitar Dimitrov  <dimitar@dinux.eu>

	* config/pru/pru.h: Mark R3.w0 as caller saved.

gcc/testsuite/ChangeLog:

2020-05-05  Dimitar Dimitrov  <dimitar@dinux.eu>

	* gcc.target/pru/lra-framepointer-fragmentation-1.c: Update test to
	take into account additional available registers.
	* gcc.target/pru/lra-framepointer-fragmentation-2.c: Ditto.

Signed-off-by: Dimitar Dimitrov <dimitar@dinux.eu>