testsuite: Unbork multilib setups using -march flags (RISC-V)

RISC-V multilib testing is currently busted with follow splat all over:

|    Schedule of variations:
|        riscv-sim/-march=rv64imafdc/-mabi=lp64d/-mcmodel=medlow
|        riscv-sim/-march=rv32imafdc/-mabi=ilp32d/-mcmodel=medlow
|        riscv-sim/-march=rv32imac/-mabi=ilp32/-mcmodel=medlow
|        riscv-sim/-march=rv64imac/-mabi=lp64/-mcmodel=medlow
...
...
| ERROR: tcl error code NONE
| ERROR: torture-init: torture_without_loops is not empty as expected

causing insane amount of false failures.

|               ========= Summary of gcc testsuite =========
|                            | # of unexpected case / # of unique unexpected case
|                            |          gcc |          g++ |     gfortran |
| rv64imafdc/  lp64d/ medlow | 5421 /     4 |    1 /     1 |    6 /     1 |
| rv32imafdc/ ilp32d/ medlow | 5422 /     5 |    3 /     2 |    6 /     1 |
|   rv32imac/  ilp32/ medlow |  391 /     5 |    3 /     2 |   43 /     8 |
|   rv64imac/   lp64/ medlow | 5422 /     5 |    1 /     1 |   43 /     8 |

The error splat itself is from recent test harness improvements for stricter
checks for torture-{init,finish} pairing. But the real issue is a latent bug
from 2009: commit 3dd1415d, ("i386-prefetch.exp: Skip tests when multilib
flags contain -march") which added an "early exit" condition to i386-prefetch.exp
which could potentially cause an unpaired torture-{init,finish}.

The early exit only happens in a multlib setup using -march in flags
which is what RISC-V happens to use, hence the reason this was only seen
on RISC-V multilib testing.

Moving the early exit outside of torture-{init,finish} bracket
reinstates RISC-V testing.

| rv64imafdc/  lp64d/ medlow |    3 /     2 |    1 /     1 |    6 /     1 |
| rv32imafdc/ ilp32d/ medlow |    4 /     3 |    3 /     2 |    6 /     1 |
|   rv32imac/  ilp32/ medlow |    3 /     2 |    3 /     2 |   43 /     8 |
|   rv64imac/   lp64/ medlow |    5 /     4 |    1 /     1 |   43 /     8 |

gcc/testsuite:
	* gcc.misc-tests/i386-prefetch.exp: Move early return outside
	the torture-{init,finish}

Signed-off-by: Vineet Gupta <vineetg@rivosinc.com>