Fix gccgo assignment error in Unix2syscall

[lechner]

Compiles with gccgo, but the test for emulated getdents fails. Related to issue #201.

[danim7]

Nice, being able to compile gocryptfs with two different compilers makes it more resilient ! Just wondering: which compiler gives gocryptfs the best performance?

[rfjakob]

Just wondering: which compiler gives gocryptfs the best performance?

I believe the default Go compiler will be hard to beat on common platforms, especially amd64.

[lechner]

@danim7, on a CPU with AES-NI, gocryptfs is much faster when built with golang-go:

AES-GCM-256-OpenSSL 	 723.23 MB/s	
AES-GCM-256-Go      	1268.90 MB/s	(selected in auto mode)
AES-SIV-512-Go      	 200.24 MB/s	

Compared to gcc-go:

AES-GCM-256-OpenSSL 	 224.06 MB/s	
AES-GCM-256-Go      	  70.11 MB/s	(selected in auto mode)
AES-SIV-512-Go      	  38.24 MB/s	

When run with kernel Linux 4.14.0-3-amd64 #1 SMP Debian 4.14.13-1 (2018-01-14) x86_64 GNU/Linux on the following processor:

Architecture:        x86_64
CPU op-mode(s):      32-bit, 64-bit
Byte Order:          Little Endian
CPU(s):              8
On-line CPU(s) list: 0-7
Thread(s) per core:  2
Core(s) per socket:  4
Socket(s):           1
NUMA node(s):        1
Vendor ID:           GenuineIntel
CPU family:          6
Model:               60
Model name:          Intel(R) Core(TM) i7-4770 CPU @ 3.40GHz
Stepping:            3
CPU MHz:             3091.832
CPU max MHz:         3900.0000
CPU min MHz:         800.0000
BogoMIPS:            6784.50
Virtualization:      VT-x
L1d cache:           32K
L1i cache:           32K
L2 cache:            256K
L3 cache:            8192K
NUMA node0 CPU(s):   0-7
Flags:               fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc cpuid aperfmperf pni pclmulqdq dtes64 monitor ds_cpl vmx smx est tm2 ssse3 sdbg fma cx16 xtpr pdcm pcid sse4_1 sse4_2 x2apic movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand lahf_lm abm cpuid_fault epb invpcid_single pti tpr_shadow vnmi flexpriority ept vpid fsgsbase tsc_adjust bmi1 avx2 smep bmi2 erms invpcid xsaveopt dtherm ida arat pln pts