crypto: arc4: use u32 instead of u8 for the variables and ctx

Changing u8 in struct arc4_ctx and variables to u32 brings about various speed-ups.
Below are tcrypt results of old 1-byte block cipher versus ecb(arc4) with u8 and ecb(arc4) with u32.

tcrypt results, x86-64 (speed ratios: new-u32/old, new-u8/old):
                  u32    u8
AMD Phenom II   : x3.6   x2.7
Intel Core 2    : x2.0   x1.9

tcrypt results, i386 (speed ratios: new-u32/old, new-u8/old):
                  u32    u8
Intel Atom N260 : x1.5   x1.4

Credits to Jussi Kivilinna
Signed-off-by: zwliew <zhaoweiliew@gmail.com>

Change-Id: I742ff35f7edcf1c0b3c07eb48992d2b1e3a5a731