feat: Add AIX process support to gopsutil

[Dylan-M]

Description

This PR adds comprehensive AIX process support to gopsutil by implementing the full process package and host metrics for the AIX platform.

This includes:

• New SignalsPending() method for retrieving pending signals for a process (with stubs for other OSes returning "Not Implemented")
• Implementation of numProcs() in the host package to count running processes via ps aux command
• Full AIX process package implementation with complete process information retrieval and metrics

Part 7 of splitting #1969: process package
Depends on #1979 #1980 #1981 #1982 #1983

Changes

• Add SignalsPending() method to retrieve pending signals for a process
• Implement numProcs() in host package for AIX using ps aux | wc -l command
• Implement AIX-specific process package (process_aix.go) with support for:
  • Process information retrieval (status, cmdline, name, executable, environment, etc.)
  • Process metrics (CPU times, memory usage, page faults, I/O stats)
  • Memory mapping information via procmap command
  • Process relationships (parent, children, status)
  • Signal handling and process control
• Add comprehensive test suite for AIX process functionality (process_aix_test.go)
• Add test data and fixtures for AIX platform testing (real procfs binary snapshots)
• Update platform-specific implementations across multiple OS modules

Testing

• All existing tests continue to pass
• New AIX process tests added with real AIX system test data
• Tested on actual AIX system (ppc64) to verify functionality
• Process count verification: numProcs() output matches ps aux command exactly
• Code formatted with gofmt
• Passes all 26 golangci-lint linters with AIX cross-compilation configuration